1. Field of the Invention
This invention relates to a tapered roller bearing used, e.g., in the shaft support section of the transmission of an automobile.
2. Brief Description of the Prior Art
Transmissions (main speed change gears) for automobiles are broadly classified into two types, the manual type and the automatic type. Further, according to the driving systems of vehicles there are front wheel drive (FWD) transaxles, rear wheel drive (RWD) transmissions, and four wheel drive (4WD) transfers (auxiliary speed change gears). These are used to speed-change the driving force from the engine and transmit it to the driving shaft or the like.
FIG. 8 shows an example of construction of a transmission for automobiles. This transmission is of the synchronous meshing type, the left side of the figure being the engine side and the right side the driving wheel side. A tapered roller bearing 43 is interposed between a main shaft 41 and a main drive gear 42. In this example, the inner periphery of the main drive gear 42 is directly formed with the outer ring raceway surface of the tapered roller bearing 43. The main drive gear 42 is supported by a tapered roller bearing 44 for rotation relative to a casing 45. A clutch gear 46 is attached to the main drive gear 42, and a synchro mechanism 47 is disposed adjacent the clutch gear 46.
The synchro mechanism 47 comprises a sleeve 48 adapted to be axially moved by the action of a selector (not shown), a synchronizer key 49 axially slidably mounted on the inner periphery of the sleeve 48, a hub 50 attached to the outer periphery of a main shaft 41, a synchronizer ring 51 slidably mounted on the outer periphery (cone section) of the clutch gear 46, and a presser pin 52 and a spring 53 which elastically press the synchronizer key 49 against the inner periphery of the sleeve 48.
In the state shown in the figure, the sleeve 48 and the synchronizer key 49 are held in the neutral position by the presser pin 52. At this time, the main drive gear 42 is idling relative to the main shaft 41. On the other hand, when the sleeve 48 is moved from the state shown in the figure to, for example, the axially left side by the action of the selector, the synchronizer key 49 is moved to the axially left side while accompanying the sleeve 48, thereby pressing the synchronizer ring 51 against the slope surface of the cone section of the clutch gear 46. Thereby, the rotational speed of the clutch gear 46 is reduced and reversely the rotational speed on the synchro mechanism 47 side is increased. And about the time when their rotational speeds synchronize with each other, the sleeve 48 further moves to the axially left side to mesh with the clutch gear 46, whereupon the main shaft 41 and the main drive gear 42 are connected through the synchro mechanism 47. Thereby, the main shaft 41 and the main drive gear 42 synchronously rotate.
In this connection, in recent years the trend of transmissions for automobiles has been directed to the use of low viscosity oils for purposes including conversion of transmissions into AT or CVT and low fuel consumption. In environments where low viscosity oils are used, if such adverse conditions as (1) high oil temperature, (2) low flow rate of oil, and (3) occurrence of release of preload simultaneously happen, surface originated flaking leading to very short life due to poor lubrication sometimes occurs in the inner ring raceway surface subjected to high surface pressure.
A differential of maximum surface pressure in the raceway surface influences the generation factor of the surface originated flaking. Therefore, reduction of maximum surface pressure is a direct and effective approach to the problem. To reduce the maximum surface pressure, the bearing size must be changed or if such bearing size change is impossible, the filling factor of rollers in the bearing must be increased. To increase the number of rollers without decreasing the roller diameter and to secure pocket spacing of the cage, it is necessary to increase the pitch circle of the cage so as to draw the cage to the outer ring side as much as possible.
As an example in which the cage is drawn until it contacts the inner diameter surface of the outer ring, there is a tapered roller bearing shown in FIG. 9 (see Patent Document 1, Japanese Laid-Open 2003-28165). This tapered roller bearing 61 is adapted to guide a cage 62 while slide-contacting the outer peripheral surfaces of the small and large diameter side annular sections 62a and 62b of the cage 62 with the inner diameter surface of the outer ring 63, and has a recess 64 formed in the outer diameter surface of the pole section 62c of the cage 62 for suppressing the drag torque, thereby maintaining the non-contact state between the outer diameter surface of the pole section 62c and the raceway surface 63a of the outer ring 63. The cage 62 comprises the small diameter side annular section 62a, the large diameter side annular section 62b, and the plurality of pole sections 62c axially connecting the small and large diameter side sections 62a and 62b and having the recess 64 formed in the outer diameter surfaces thereof. And, there are provided a plurality of pockets for rollably receiving tapered rollers 65 between adjacent pole sections. The small diameter side annular section 62a is provided with an integrally extending flange 62d on the inner diameter side. As compared with the conventional type in which the cage and the outer ring do not contact each other, the tapered roller bearing of FIG. 9 is capable of increasing the roller filling factor, thus making it possible to prevent early breakage due to excessive surface pressure on the raceway surface.